Thermodynamics for charge density wave phase transition in bulk 1T-TaS2 from first-principle calculation

Bulk 1T-TaS2 shows a Star-of-David-shape charge density wave (CDW) lattice distortion via electron-phonon interaction and two-step phase transition with changes in electron conductivity. Although this behavior has been experimentally observed since 1964, the experimental and theoretical understanding of its microscopic mechanism has not been completely developed until now. Here, we investigate the possible microscopic mechanism of CDW phase transition in bulk 1T-TaS2 through ab initio calculation with machine learning force field technique. Our analysis from extensive static total energy and molecular dynamics calculations reveals that the CDW shifting via CDW breathing motion leads to the stacking order randomization at 150K < T , and its deformation is achieved with enough thermal energy (T > 350K). With density functional theory plus Hubbard

U potential for generalized orbitals calculation, we verify possible metallic stacking configurations contributing to the metallic nature. Our study provides the microscopic mechanism of two-step CDW phase transitions, which involves the stacking order and CDW deformation in 1T-TaS2.